Prepare a medium-hot fire in a charcoal grill with a cover, or preheat a gas grill to medium-high.

To make the spread, combine the cream cheese, cheddar cheese, and green chiles in a small bowl. Stir until well combined. Cover with plastic wrap and set aside.

To make the patties, place the chuck a large bowl and add the onion, garlic, chile powder and salt. Combine well, handling the meat as little as possible to avoid compacting it. Form into 6 patties, cover with plastic wrap, and refrigerate until ready to grill.

To make the salsa, peel the husks off the tomatillos and rinse well; pat dry with a paper towel. Grill the whole tomatillos directly on the grill rack, turning frequently, until skins are blackened, about 3 to 5 minutes. Remove from grill. When cool enough to handle, remove the stems and stem base and slice in half. Place the grilled tomatillo halves and the water in a blender or food processor; pulse a few times, until somewhat smooth, leaving some texture in the mix. Pour tomatillo mixture in a medium bowl. Add the onion, avocado, garlic, serrano chile, cilantro, salt and cumin. Stir until well-combined. Cover with plastic wrap and set aside.

Brush the grill rack with oil. Place the patties on the rack, cover, and cook for 4 to 5 minutes per side, until the desired doneness is reached. During the last few minutes of cooking, place the buns cut sides down on the outer edges of the rack to toast lightly.
To assemble the burgers, generously spread the green chile cheese spread over the cut sides of the bun tops. Place a patty on each bun bottom and top with equal portions of the salsa. Add the bun tops and enjoy.